Graphene can be synthesized in different forms depending on the manufacturing method and required application. Graphene flakes can be obtained starting from graphite in the same way as it was first isolated and its properties determined back in 2004. The so called micromechanical exfoliation technique renders individual and isolated single crystal monolayer graphene flakes on top of a substrate such as silicon or more specifically silicon with a thin silicon oxide layer on it in order to allow visualizing the individual flakes using a standard optical microscope. These individual and isolated flakes of graphene have been shown to have extremely high charge carrier mobility, high thermal conductivity, excellent mechanical properties, high transparency, flexibility, etc. However, the main problem with this manufacturing method is that it produces a handful of flakes that could be suitable for basic research but not for industrial applications.
The expansion of graphite has been known for a number of years and the first patent related to this material dates back to 1913, US1137373. However, it was not until the late 1960s that a more considerable number of patents related to expanded graphite started to be published, U.S. Pat. No. 3,323,869, GB1186727 and U.S. Pat. No. 4,915,925. Expanded graphite is typically produced using acids and oxidizing agents as intercalating materials followed by an expansion process at high temperatures (around 1000° C.) for short periods of time. However, these processes produce slightly thinner graphite material but not monolayer graphene flakes. More recently, in U.S. Pat. No. 7,550,529 graphite nanoplatelets were produced via the expansion of natural graphite. In this patent, graphite is expanded by first intercalating an acid (sulfuric acid) and an oxidizing agent (nitric acid) followed by an expansion step using microwave or radio frequency (RF) wave treatment. This method produced slightly thinner graphite with considerably lower lateral flake dimensions than the starting graphite material. However, there was no evidence of the production of monolayer graphene flakes. Furthermore, no evidence was provided about the chemical functionalization of graphite in order to obtain covalently functionalised graphene oxide flakes.
In U.S. Pat. No. 8,114,375 patent, intercalated graphite oxide was subjected to two different thermal treatments in order to obtain the expanded material. The starting graphite is intercalated with an acid and an oxidizer prior to the thermal treatments. The first thermal treatment is undertaken at 600° C. while the second one at around 1000° C. As a consequence the final material has an oxygen content of around 5 wt %. Furthermore no evidence of the final thickness of the material is provided and it can be assumed that relatively thick graphite like material is produced using this method.
In U.S. Pat. No. 7,658,901 patent, thermally exfoliated graphite oxide is produced where it is demonstrated that the starting graphite material has been subjected to a certain chemical functionalization especially prior to the high temperature treatment. This patent relates to the production of thermally exfoliated graphite oxide where the graphite is intercalated with an acid and an oxidant followed by a thermal treatment at high temperatures up to 2000° C. However, after the thermal treatment most of the functionality disappears, almost all the oxygen containing groups are eliminated due to the high temperature treatment, which moreover is very expensive in terms of thermal energy.
In yet another patent U.S. Pat. No. 8,524,067 graphite was intercalated with carboxylic acids (formic or acetic acids) followed by an electrochemical reaction, thermal treatment and mechanical shearing. The process could be repeated several times until the thickness of the flakes was <30 nm. No clear evidence was provided about the amount and yield of monolayer flakes that could be obtained using this process. In addition due to the many processing steps this method could be deemed quite inefficient. No covalent functionalization of the graphite flakes was achieved.
Similarly in U.S. Pat. No. 8,747,623, graphite was intercalated with an acid and an oxidizing agent and immediately it was treated to microwave irradiation. Again no clear evidence was provided about the amount and yield of monolayer flakes that could be obtained using this process. The oxygen content of the flakes was below 5 wt %.
In WO 2010/042912 patent application, the preparation of graphite oxide is explained. Graphite is oxidized in concentrated sulphuric acid, sodium nitrate and potassium permanganate. Once the reaction is completed the mixture is purified in order to obtain the graphite oxide material. Since this document does not mention any thickness for the produced flakes and no exfoliation step is disclosed, it can be assumed that no graphene oxide monolayer flakes can be produced using this method.
There are other reports related to the production of graphite oxide that date back to 1898 where Staudenmaier published a method to produce graphite oxide starting from nitric acid, sulfuric acid and potassium chlorate. However, it had clear disadvantages such as long reaction times (1 week) and the use of explosive reactants. In addition, no monolayer flakes were isolated using this method, only thick graphite oxide flakes were obtained.
In 1958, Hummers and Offeman reported a method to synthesize graphite oxide using concentrated sulfuric acid, sodium nitrate and potassium permanganate. They added sodium nitrate instead of nitric acid in order to avoid the use of a very corrosive acid. Again no monolayer flakes of graphene oxide were isolated and characterized using this method, only thick graphite oxide fakes were achieved.
In U.S. Pat. No. 6,596,396 patent a manufacturing process based on a modified Hummers's method is presented in order to synthesize thin particles of graphite oxide. The described method uses concentrated sulfuric acid, sodium nitrate and potassium permanganate to produce graphite oxide. The reaction to produce of graphite oxide takes more than five days and therefore it is not a very cost-effective process. In addition, these 5 days do not include the purification process. Again the sodium nitrate is considered as a crucial component in order to obtain the graphite oxide.
The purification process is also extremely long in the U.S. Pat. No. 6,596,396 patent, where the graphite oxide dispersions are left to settle for over a day several times, extending considerably the purification process time. In addition acidic and oxygen peroxide containing mixtures are used in the cleaning procedure making the process environmentally unfriendly.
More recently, Marcano et. al. (ACS Nano 2010, 4 (8) 4806-4814) described a new method for the synthesis of graphene oxide based on a modified Hummers method. In this article the sodium nitrate is replaced by phosphoric acid and the amount of potassium permanganate amount is duplicated.
Other manufacturing methods that use graphite as a starting material are based on the so-called liquid phase exfoliation technique. In this case there is no prior oxidation step of the graphite and the exfoliation of the flakes is generally obtained via ultrasonic waves. The exfoliation of graphite is undertaken in liquid media either in the water containing surfactants or in thermodynamically favourable solvents for graphite.
In U.S. Pat. No. 7,824,651 graphite is dispersed in a liquid medium containing a surfactant or dispersing agent and subjected to ultrasonication. The thickness of the flakes after the exfoliation process is claimed to be below 100 nm however, no evidence is provided about the efficiency and the real thickness of the produced flakes. No oxidation step is undertaken therefore no covalent functionalization of the graphite flakes.
WO 2014/140324 A1 discloses a process for exfoliating untreated 3-dimensional material to produce a 2-dimensional material by using a high shear mixer.
The main drawback with direct exfoliation techniques of graphite in liquid mediums is their extremely low yield and very small quantities of monolayer flakes that are obtained.
In the US 2013/0302593 patent application a similar procedure is mentioned to exfoliate MoS2, MoSe2, WS2, BN and similar type of materials. In this case the exfoliation is undertaken in water-surfactant solutions using ultrasound and centrifugal force to exfoliate them.